WWYD: Radian vs RSTV2 vs Harmony folding booster?

Will be flying with newly 5yo twins later this month. Normally they ride in FR85 in main car & Radian XTSL FF in secondary car. We will have one connecting flight, traveling from CA to IA. They've never been in a booster before, but typically are not the squirmy type. We anticipate a half hour ride from airport to our destination in a "standard size" rental. The back seat will have to accommodate an adult and the 2 kids. DD rarely sleeps in a car seat, but DS does.

Obviously taking the Radian would be the no cost solution, would provide restraint in-flight & is slim, but weighs a ton. I do own additional carry straps so it can be carried like a backpack. I wasn't really considering the FR85 just due to size. My DH only tolerates my car seat anxieties up to a certain point (eg he turned the twins FF in the Radians @ 2.5 y--I ended up purchasing the FR85 for primary & moving the Radians RF to my car to replace the Avenues they were outgrowing--they just outgrew RF in Jan of this yr, DD by weight & DS by height). I don't think he'd be happy lugging the Radians through the airport, or dealing with installing twice on the plane.

So, looking for other options, it seems like RSTV2 or perhaps Harmony folding? DD doesn't meet the minimum height for the Boostapak. At their current size, (DS is 44" and about 43-44 lbs, DD is 42" and about 45lbs), I would hope the lap belt on the plane would fit . We are going only for the weekend, so it would be nice to travel only with carry-ons if possible. The kids have flown only once before @ age 13 mo, and were in RF Avenues on a 6 hour nonstop flight. I don't know the size of the planes we'll be on, but it wouldn't surprise me if the "carryon" ends up being tagged and placed in cargo . "1 small carry-on bag, which will be tagged, placed in the cargo bin and returned to you plane-side or on the jetway when you arrive". Is that in anyway different than gate checking an item???

Hmm, maybe I've answered my own question? I don't see how there would be any concerns with the integrity of the RSTV2 if stored in a carryon that ends up in the hold? Will be using US Airways to destination & Skywest/United on way back. Or would one just hand carry the RSTV like an article of clothing (to save space in a carryon?). It should fit under the seat on a plane, right?

Cost is not an issue. I live close to a Walmart if I were to get a Harmony, & I know I can get an RSTV2 at Diapers.com.

Are you doing a lot of car travel at the destination? If not, I think the easiest thing for you would be two backless boosters, which should be small enough to be a carry-on item on almost any plane. (I've been on a few smaller aircraft where even typical "carry-on" size items like small rollaboards can't fit in the overhead bit, so they are gate-checked as you noted... but I think a backless booster would still fit under the seat in front. Perhaps the Bubble Bum if you're really concerned about portability.)

You would need a little bit of time to get the kids booster-ready, but it sounds like they are old enough at age 5, and large enough for the typical 40-lb. minimum. Plus you'd have an adult in the backseat to give them reminders about proper position, and it's a short ride of 30 minutes.

Kids over 40 lbs. are also OK on the plane with just the lapbelt per FAA recommendation.

Now, if you're doing a lot of driving at the destination and one of the kids is a car-napper and neither of them have a lot of practice with boosters, I might consider something else. The Radians are heavy but they are manageable if you have a wheeled cart. I don't have experience with the RSTV but there are folks who swear by it.